Measurement of Specific Diffusing Capacity (DL/VA) in Critically Ill Patients

  • Edith Rosenberg


The idea of measuring the specific diffusing capacity of the lungs for carbon monoxide (DL/VA) as an index of the condition of the “alveolar-capillary membrane” was introduced in 1915 by M. Kroghl. She showed that during short periods of breath-holding with the glottis open the removal of CO from alveolar gas by the hemoglobin was a first order reaction and proposed that the time constant of the exponential decay of alveolar CO during breath-holding be used as a measure of the size of the “alveolar capillary membrane”. This time constant
$$=\frac{1}{t}~~.1n{{\frac{{{F}_{{{A}_{CO}}}}}{{{F}_{{{A}_{CO}}^{\prime \prime }}}}}^{\prime }}~\frac{1}{B-47}~~where~t~~is~the$$
period of breath-holding,\({{F}_{{{A}_{CO}}}}^{\prime }\)is the alveolar CO concentration at the beginning of the period of breath-holding,\({{F}_{{{A}_{CO}}}}^{\prime }\)is the alveolar CO concentration at the end of the breath-holding period and B is barometric pressure. The term specific diffusing capacity (DL/VA) was introduced by Ayers et al in 19752. It is directly proportional to Krogh’s time constant i.e.
$${{F}_{{{A}_{CO}}}}^{\prime }$$
But the use of DL/VA or Krogh’s time constant in this manner assumes that the time required for CO to combine chemically with hemoglobin is negligible compared to the time required for CO to diffuse across the alveolar-capillary membrane. In 1957 it was shown that the rates of the two processes, diffusion and chemical combination with hemoglobin are of the same order of magnitude3. Consequently DL/VA is only an index of the condition of the membrane, not an actual measurement.


Lung Volume Total Lung Capacity Helium Concentration Alveolar Volume Pulmonary Capillary Blood 
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Copyright information

© Plenum Press, New York 1983

Authors and Affiliations

  • Edith Rosenberg
    • 1
  1. 1.Department of Physiology and Biophysics College of MedicineHoward UniversityUSA

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